Once the Republicans vowed to set aside precedent in a nakedly partisan move, why wouldn't Obama rub it in by choosing a candidate that they couldn't reasonably object to ? Why give them an excuse for fighting him ? If the appointment is "about the election" why not help the Republicans make themselves look bad ?
Obama has been pretty moderate throughout his terms, so I'd have expected him to go for a moderate anyway - certainly so long as the Republicans run the Senate. But the Republicans just walked into this one.